It's less about the travel and more about the ethos that "Planescape is always right" in how it presents itself as an omni-setting that subsumes all other settings within itself.
Originally that was because Gary Gygax only invented one cosmology and didn't think you needed more. It had nothing to do with Planescape.
Planescape actually started the shift away from that and was more about a theme that was equal parts "Everyone is right!" and "No one really knows!" The
Manual of the Planes was clear that this was the one true way and Planescape followed that mechanically because there simply wasn't any other options until 3e. But thematically, in the flavor of the setting, the entire point of Planescape was about those equal parts "Everyone is right!" and "No one really knows!" Sure, it liked to joke around with the term "clueless prime," but if you pay attention, a lot of the time it's used ironically to show how clueless the planar NPC actually is.
Sure, Planescape unified all the settings, but so did Spelljammer - both with very different styles, and both entirely preserving the original settings. Being the connective tissue doesn't mean it's suddenly everything and subsumes it all. Highways unify cities, but that doesn't make every city suddenly just a highway. Each city still has it's own character, and the highways themselves can even have their own character as well.
Planescape was always presented as a unique setting that occurred in the Planes rather than just any adventures in the Planes (just set any version of the Manual of the Planes next to any Planescape product, and the differences can be pretty clear if you look). Spelljammer was also presented as a unique setting that occurred in space. Both can touch other settings and are easily accessible back and forth, but saying that just because Spelljammer linked all settings, it subsumes them and all settings are just Spelljammer completely misses everything that makes Spelljammer unique (as well as all of those settings unique). It's the same with Planescape. Both were presented as ways to
unite the settings, not replace them.